1. Field of the Invention
The present invention relates generally to acoustic emission monitoring systems. More particularly, the subject invention pertains to an acoustic emission monitoring system for locating a source of acoustic waves in a structural member such as an aircraft to detect structural defects therein. Structural defects such as stress cracks emit acoustic and stress waves which propagate outwardly therefrom. By monitoring the structure for acoustic emissions, the existence and location of defects such as stress cracks can be determined.
In greater detail, the present invention relates to a method of acoustic emission source location by reverse ray tracing which allows the structure along the intervening path of a detected wave and the most probable perturbations of the detected wave to be taken into account. The total of all of the iterative changes taken by the wave are calculated, providing a very accurate measurement of the path and the true location of the acoustic emission signal source.
The present invention has particular applicability to the testing of the structural integrity of aircraft. When minor flaws develop in an aircraft structure, elastic acoustic and stress waves are generated by the flaws, and it is important to be able to locate the source of the acoustic emissions so that appropriate repairs can be made. The present invention offers that capability.
The problem of an aging aircraft fleet is a cause for concern for our society, which has the busiest commercial air traffic system in the world and also the largest military aircraft inventory. The assured airworthiness of these aircraft places a burden on all involved, from the industries that build and maintain them, through to the government regulatory agencies responsible for maintaining adequate oversight thereof. Moreover, because of economic and political factors, military and commercial carriers are retaining their equipment longer than usual, putting a further strain on aircraft which are reaching the waning hours of their useful service life.
Furthermore, the dynamics of service-induced loads on an aging aircraft are not always the same as those experienced by a young, robust aircraft of the same family of aircraft. Commonly, in aging aircraft one finds that the repair and modifications themselves shift the strain loads to new areas, sometimes to uninspected areas, only noticed as a result of their failure. A system is required to perform constant assessments on the structural integrity of aging aircraft and also to inspect details in the complex structures of modern aircraft.
Acoustic emission monitoring is useful for global detection and location of cracks, impacts, delaminations and other fatigue damage in large scale structures such as aircraft because of the remote sensing capability of an acoustic emission monitoring system, and its use of a contiguous structure as a propagating medium for damage induced stress waves.
2. Discussion of the Prior Art
Acoustic emission monitoring is frequently used in aircraft to detect structural defects such as stress cracks which emit acoustic and stress waves which propagate outwardly therefrom. By monitoring the acoustic emissions, the existence and location of defects such as stress cracks can be determined.
The technological field of acoustic emission monitoring has several problems which must be overcome before a useful result is obtained from its application. One major problem is determining the precise location of the source of the acoustic emission signals.
To locate an acoustic emission signal source, some prior art systems require four piezoelectric sensors placed a measured distance from each other and surrounding the expected source of signals. As a stress wave propagates across a structure, it passes by each of the monitoring sensors. As each sensor is hit, a clock circuit is triggered until all of the sensors in the array have detected the passing stress wave, and the arrival time for each sensor is known. To deduce the origin of a stress wave, the differences in the arrival times are calculated for each set of two sensors in the grouping. The differences in arrival time, or .DELTA.t, for each set of two sensors represent a set of points. A plot of these locations generates the characteristic curve of a hyperbola, along which are the innumerable points from which the signal may have emanated. So, using this approach, two sensors tell little of the specific origins of signals. At least a third sensor and an additional .DELTA.t is required to locate the signal source by the corresponding overlapping hyperbolas.
In order for any group of sensors to be able to cover a large area of a structure and also use a practical number of sensors, the sensors are typically deployed as far apart as possible. But in the intervening space between the sensors, the stress waves can be modulated by the complex structure so that each sensor may detect a wave with modified waveform characteristics which can distort the arrival times. Since conventional acoustic sensors cannot discriminate direction, they are prone to detect and accept signals from simultaneous sources, and interpret them as the same stress wave hitting the other sensors in the group. These disparities multiply as the errors of each arrival time are added into the location equation.